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Comparison Between Hydrogen and Syngas Fuels in an Integrated Micro Gas Turbine/Solar Field with Storage

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Comparison Between Hydrogen and Syngas Fuels in an Integrated Micro Gas Turbine/Solar Field with Storage energies Article Comparison between Hydrogen and Syngas Fuels in an Integrated Micro Gas Turbine/Solar Field with Storage Maria Cristina Cameretti, Alessandro Cappiello, Roberta De Robbio * and Raffaele Tuccillo Department of Industrial Engineering, University of Naples Federico II, 80125 Naples, Italy; [email protected] (M.C.C.); [email protected] (A.C.); raff[email protected] (R.T.) * Correspondence: [email protected] Received: 9 June 2020; Accepted: 3 September 2020; Published: 12 September 2020 Abstract: In recent years, the use of alternative fuels in thermal engine power plants has gained more and more attention, becoming of paramount importance to overcome the use of fuels from fossil sources and to reduce polluting emissions. The present work deals with the analysis of the response to two different gas fuels—i.e., hydrogen and a syngas from agriculture product—of a 30 kW micro gas turbine integrated with a solar field. The solar field included a thermal storage system to partially cover loading requests during night hours, reducing fuel demand. Additionally, a Heat Recovery Unit was included in the plant considered and the whole plant was simulated by Thermoflex® code. Thermodynamics analysis was performed on hour-to-hour basis, for a given day as well as for 12 months; subsequently, an evaluation of cogeneration efficiency as well as energy saving was made. The results are compared against plant performance achieved with conventional natural gas fueling. After analyzing the performance of the plant through a thermodynamic analysis, the study was complemented with CFD simulations of the combustor, to evaluate the combustion development and pollutant emissions formation, particularly of NOx, with the two fuels considered using Ansys-Fluent code, and a comparison was made. Keywords: agricultural product; cogeneration; exergetic analysis; hydrogen; micro gas turbine; syngas 1. Introduction Micro Gas Turbines (MGT) represent a promising solution for distributed cogeneration due to important energy saving and reduction in pollutant emissions which characterize this type of power plant. Furthermore, we are witnessing a great variety of gas turbine-based hybrid power plants that combine two or more power generating devices and make use of the synergism to generate maximum power and heat recovery, thus offering high efficiencies. In this regard, for small-scale, distributed energy application, hybrid systems based on the integration between solar field and gas turbine (GT) technologies are recognized as a very interesting solution [1–6]. In addition, the flexibility of gas turbines offers the possibility to use alternative fuels that can further reduce the carbon footprint and energy demand. Particularly, two type of fuels are receiving growing attention—i.e., syngas and hydrogen—since they can be produced from wind or solar power and from any hydrocarbon-based feedstock gasification or pyrolysis of biomasses [7–12]. In particular, the syngas, consisting of a mixture of methane, carbon monoxide, hydrogen and a significant amount of inert gases, such as carbon dioxide and nitrogen, with percentages of each species being dependent on the gasification process used for its production, shows a lower LHV with respect to the natural gas. Consequently, the fuel change imposes a substantial increase in the fuel mass flow rate in order to meet the same turbine inlet temperature (TIT), leading to an off-design operation of the whole Energies 2020, 13, 4764; doi:10.3390/en13184764 www.mdpi.com/journal/energies Energies 2020, 13, 4764 2 of 24 system and to a change in the reacting mixing quality in the combustion chamber, which in turn requires a trade-off between combustion efficiency, pollutant formation and overall system efficiency [9]. Additionally, another characteristic issue related with use of syngas fuel in MGT combustors is a poor combustion completion. On the other hand, the use of hydrogen fuel offers several appealing features, such as a high LHV and a carbon-free composition, although some other critical aspects are associated with its use in MGT combustors, namely the increase in NOX production due to the higher flame temperature and the risk of flashback [10]. The use of these new fuels has been investigated by several researchers, who tried to address the abovementioned issues. Concerning syngas fuel, Abagnale et al. [7] investigated the performance of a 30 kW MGT annular combustor when fueled with two type of syngas and found that syngas from biomass gasification, despite a good combustion, can lead to an increase in NOX formation. Cameretti et al. [8] investigated the suitability of liquid and gaseous fuels for use in a lean premixed MGT combustor and also assessed the potential of external EGR for NOX reduction. Published literature also reports other promising solutions for combustion completion improvements and NOX mitigation, such as the increase in pilot injection rate and modification to the pilot location, respectively, [9]. Mărculescu et al. [13] analyzed the change in performance when the MGT is adapted to burn alternative low-quality gas fuel produced by biomass gasification with heating values 3 to 5 times lower than methane. At constant heat flow rate in the combustion chamber, the gas flow rate increases to keep the temperature of the flue gases constant. Then, the gross electric power increases but the net electric output decreases compared to methane use. Nicolosi et al. [14] assessed the effect of the variation of the operating conditions on the performance of the recuperator and, therefore, of the whole MGT. The use of alternative fuels with low LHV shifts the operative points of the turbomachines; in general, compression ratio is reduced, as is the flow rate of the compressor. Therefore, attention must be paid to the compressor stall limit. The recuperator shows a slight variation in the temperature of the fluids, but a higher efficiency is recorded as the flow rate is typically reduced and a better heat recovery performance can be obtained. Hasini et al. [15] performed a CFD investigation, using ANSYS Fluent, of flow, combustion process and pollutant emission using natural gas, liquefied natural gas and syngas of different composition on a combustor can-type. The prediction of pollutant species concentration at combustor exit shows significant reduction in CO2 and NOX for syngas combustion compared to conventional natural gas and LNG combustion. Ammar et al. [16] examined four syngas fuel compositions which differed by H2/CO ratios. The volume of CO2 at the exhaust decreases with the increase in hydrogen content, while the NO emissions increase as the hydrogen content increases. However, syngas fuel has lower emissions of NO and CO2 and higher emissions of CO than those from natural gas fuel at the same operating conditions. In [17] it is demonstrated that the reactivity of the syngas mixture is governed by hydrogen chemistry for CO concentrations lower than 50% in the fuel mixture, while for higher CO concentrations, an inhibiting effect of CO is observed. The syngas supply and combustion efficiency of the MGT Capstone C30 were already experimentally investigated in [18,19], by testing different compositions that have various hydrogen and carbon monoxide ratios. It was found that the high amount of hydrogen content in syngas leads to an increase in the combustion efficiency and emits more NOx emissions. Indeed, higher hydrogen content elongates the flame because of the presence of more H radicals. This H radical promotes the chain branching and chain propagation, enhancing the reaction zone length and flame speed. Thus, the flammability limits are extended so that the flame is able to achieve stability at leaner conditions. On the other hand, CO, emissions are released when the combustion efficiency is low and increase with the CO content in the syngas. Hydrogen fuel is considered another appealing alternative to fossil fuels, potentially capable of relieving global CO2 emissions. Pambudi et al. [20] analyzed the impact of hydrogen in the power generation sector by comparing a base scenario without hydrogen with one in which hydrogen Energies 2020, 13, 4764 3 of 24 substitutes part of the fossil fuel supply from 2020 onwards. The use of hydrogen would result in substantially less CO2 being released into the atmosphere, leading to emission reductions of nearly 60% by 2050. However, the percentage of hydrogen to the energy supply must be around 10% due to the increased NOx emissions. The suitability of the use of CH4/H2 blends in MGT was investigated in various ratios both in lean premixed and in RQL combustors by Tuccillo et al. [10], who found that the lean premixed combustor type appears more prone to flashback at lower H2 concentrations and that RQL combustors offer a better chance for NOX control via the separate injection of methane and hydrogen. However, the increased combustor outlet temperature might be a problem from a turbine blade thermal resistance point of view, as well as combustor design modifications in the dilution zone, which might be necessary [11]. In this regard, the design and construction of a 100 kW pure hydrogen fueled MGT is illustrated in [21]. The progressive optimization of the compressor–combustor system is achieved starting from a full thermodynamic cycle analysis and though CFD simulations in steady and transient conditions. In [22], steam injection (STIG) is proposed as a solution to limit the drawbacks caused by the use of alternative fuels in MGT. A proper injection strategy is evaluated to enhance the electric power by 24% and the electric efficiency up to 29%. Moreover, steam injection in the combustion chamber, allows for dramatically reduced CO and NOx emissions. In particular, the higher CO emissions can be reduced by injecting the majority of the steam in the dilution zone, allowing higher temperatures, which enhance the CO oxidation, while the NOx emissions can be reduced by injecting part of the steam in the pilot zone, where the majority of the NOx forms.
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